Imperative versus declarative programming

Imperative programming concerns itself with how something is accomplished, while declarative programming concerns itself with what we want accomplished. It's difficult to see the difference between these so it's best to illustrate them with a simple program:

function getUnpaidInvoices(invoiceProvider) {
  const unpaidInvoices = [];
  const invoices = invoiceProvider.getInvoices();
  for (var i = 0; i < invoices.length; i++) {
    if (!invoices[i].isPaid) {
      unpaidInvoices.push(invoices[i]);
    }
  }
  return unpaidInvoices;
}

This function's problem domain would be: getting unpaid invoices. That is the task the function has and it is what we want to achieve within the function. This particular function, however, concerns itself a lot with how to achieve its task:

• It initializes an empty array
• It initializes a counter...